A significant challenge of permanent magnet DC brush-type motors is to achieve different speeds of operation. Wound Field type motors generally have speed controlled by altering the field flux. This is done by changing the current or the number of coil turns of the field winding. Since permanent magnet motors have a constant field flux, they cannot achieve speed control by field flux variation.
Often, permanent magnet motors used in automotive applications require the use of more than one speed, usually a lower speed for general purpose operation and a maximum speed for worst case operation. For example, multiple speed operation of a vehicle cooling system module provides a more optimized engine temperature and operation, which consequently contributes to improved fuel economy.
For permanent magnet DC brush-type motors, historically lower speeds (multiple speed operation) have been achieved by the following methods:                Adding a resistor in series with the motor        Switching out brushes (lap wind motor)        Dual armature winding with dual commutator        Adding an additional 3rd brush (short out coils)        External or internal electronic control comprised of but not limited to:        SSR, (Solid State Relays)        Linear Control        PWM, (Pulse Width Modulation)        
FIGS. 1A, 1B and 1C show a magnetic circuit of 2 pole, 4 pole and 6 pole conventional brush-type permanent magnet DC motor, respectively. The lines of flux are created such that they leave the North Pole magnet and enter into the air gap. The flux lines enter into a lamination stack of a rotor of the motor and travel along the shortest possible path to the magnet of opposite polarity. The lines of flux again travel through the air gap and into the magnet of opposite polarity. The lines of flux then leave the second magnet and return back to the original magnet. Hence, the lines of flux are understood to operate in a closed loop circuit. The flux available in the air gap between the magnet and the rotor lamination determines the operating characteristics of the motor.
The magnetic flux can be produced using either a wound field stator or permanent magnet stator. FIG. 2 shows the magnetic circuit of a 2 pole wound field motor where the (N) coils of wire around a pole shoe 10 produce the magnetic flux. One concern is that motors with permanent magnets have a constant level of field flux (or stator flux) and hence only operate with one speed. The advantage of a wound field stator is that various windings can be applied to change the level of field flux or stator flux. The disadvantage of using a wound field stator is that in order to generate sufficient field flux, significant numbers of coil turns are required for each field coil. This requirement causes the stator to be bulky, heavy and relatively expensive.
In examination of conventional speed control methods another method to vary the motor flux is to combine permanent magnets with field wound coils to produce the desired level of flux.